This invention relates to improved amphoteric starch derivatives containing both cationic substituent groups and anionic phosphate groups. The invention also relates to a process for preparing these amphoteric starches and their use as wet-end additives showing improved drainage in the manufacture of paper.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic material, which may be derived from natural sources as well as from synthetics such as from mineral fibers such as asbestos and glass. Also included are papers made from combinations in cullulosic and synthetic materials. Paperboard is also included, within the broad term "paper".
It has been known to add various materials, including starch, to the pulp, or stock, during the papermaking process, prior to the formation of the sheet. The purpose of such additives has been mainly to bind the individual fibers to one another, thus aiding the formation of a stronger paper.
In the case of those papers which contain added pigments, such for example as titanium dioxide, it has been known to add materials to the pulp, or stock, for the specific purpose of retaining a greater proportion of such pigments in the paper (rather than have them drain off in the water that is removed during the formation of the sheet). Such additives are often referred to as "pigment retention agents".
Anionic and cationic starches as well as amphoteric starches have long been used as additives in papermaking for their contributions to strength and pigment retention in the paper. See, for example, U.S. Pat. Nos. 3,459,632 to Caldwell et al. and 3,562,103 to Moser et al.
In recent years, in an effort directed to improving machine speeds, reducing drying time and generally improving costs, paper makers have begun to focus on the efficiency of water removal or drainage during the sheet-forming process. See, for example, the article by K. W. Britt in TAPPI Journal, Jan. 1984, p 102-103 and the article by A. M. Springer et al., TAPPI Journal, Feb. 1984 p 104 108. In ordinary papermaking operations employing Fourdrinier type machines, the pulp furnish or stock is fed from a headbox onto a wire screen where the web is first formed. Water is drained from the stock by gravity and by vacuum suction, and then by pressing. Drainage efficiency is affected by a number of factors including the composition and pH of the furnish. It can be understood that even minor improvements in drainage efficiency can have significant beneficial effects on the economics of paper manufacturing due primarily to the large volumes of paper which are produced.
While starch additives of the prior art used for strength and pigment retention also show some favorable effects on improving drainage performance, we have found that the use of the amphoteric starch additives described hereinbelow, meeting specified criteria particularly with respect to bound phosphorus, surprisingly provide improved drainage with no untoward effects on strength or pigment retention. Testing results have shown that the amphoteric starch additives of the present invention will typically improve drainage by 20-30% over a representative commercially used amphoteric starch additive.
Accordingly, one object of the invention is to provide a process for preparing amphoteric starches meeting criteria with respect to molecular weight, bound phosphorus, cationic degree of substitution, and the ratio of anionic to cationic groups.
Another object of the invention is to provide an improved amphoteric starch containing cationic groups and anionic phosphate groups meeting criteria with respect to molecular weight, bound phosphorus, cationic D.S., and the ratio of anionic to cationic groups.
A further object of the invention is to provide an amphoteric starch for use as a wet-end additive showing improved drainage in the manufacture of paper.